Radar sensor technology is now very widespread in process automation and monitoring. Radar technology enables the speed, position and presence of a material reflecting microwaves to be measured directly.
In the power station field there is the requirement for optimum dimensioning of the radial gap on turbines. To this end it is necessary to monitor the radial gap. The definition of “radial gap” is expressed by the space between the outer end of turbine blades and the turbine housing. If this gap is large during the operation of the turbine, a part of the working gas, of which the energy should actually be taken up by the turbine blades, will flow as a secondary flow and thus as a loss flow through this gap and will reduce the efficiency of the turbine. With large turbines especially, such deteriorations in efficiency, even of only a few tenths of a percent, are clearly reflected in the fuel costs.
It is therefore of commercial and technological interest to be able to determine with great accuracy the radial gap of turbines during operation. In the past contact-based measurement processes were generally employed. Such measurement methods are nowadays already at a high level of development. At the same time however inherent disadvantages exist with these methods which should have been rectified in the interim. There is a natural interest in non-contact measurement methods in this area.
Of the known methods there are essentially three different systems to be mentioned which deal with determining the gas turbine radial gap during operation.
The oldest method uses a wear pin made of a soft material which is introduced into the turbine housing until it contacts individual turbine blades. This mechanical method has the disadvantage of high wear and inadequate measurement options. In this case only the blade projecting furthest outwards, i.e. the blade with the minimum radial gap is detected.
In a further development there is the above-mentioned system with electromechanical actuator. A disadvantage associated with said method lies in using a component subject to wear and the inherent necessity of using mechanical elements.
A fundamentally different measurement principle is based on a capacitive proximity sensor which eliminates a few of the disadvantages of the previously mentioned systems. With this method measurement errors occur which are also a function of the blade geometry. In addition the possible measurement accuracy is very limited.